Political scientists might say there is nothing wrong with this. Parties ought to stand for something. The Democratic Party's tent was bound to be unstable when it stretched so far as to encompass Southern racists and Northern African Americans. Those who used to be called Rockefeller Republicans -- believers in activist government -- may feel more at home as Democrats. There is value to clarity in such things.

But there are dangers, too. The world is complicated, and an electorate so diverse in geography, race, class and beliefs can't be shoehorned into two fixed templates. There is no particular reason why all advocates of fiscal restraint should also oppose abortion rights, or why supporters of a progressive tax code should necessarily favor restrictions on gun ownership. The more litmus tests are imposed, the greater the number of voters who will find themselves politically homeless.

Moreover, the ideologies that the parties seek to impose risk turning the United States into the next Greece. It is only slightly a caricature to say that today's Republicans offer tax cuts as an answer to all problems in all situations, no matter the present level of taxation, and that Democrats defend every entitlement program continuing into eternity, no matter how unaffordable. The only ground for "compromise," as we have seen, then is to cut taxes and raise spending, dooming the country to unsupportable debt. Both Mr. Bennett and Ms. Lincoln have come under attack for supporting the Bush administration's rescue of the nation's economy. Voters may have legitimate gripes against these incumbents, but this isn't one of them; its resonance suggests that one casualty of the mutual purges may be responsible government.

For many party cleansers, working across party lines constitutes treason. We agree that elected officials ought to be guided by principles that they are willing to fight for. But we also see a difference between fidelity to principle and dogmatism. If Republicans cannot accept that Democrats may make some reasonable arguments, and vice versa, then nothing will get done: no energy policy, for example. Mr. Bennett worked with Democratic Sen. Ron Wyden of Oregon to develop a bipartisan health bill that responded to each party's noble aspirations: Republican commitment to economic competition and individual choice in the service of Democratic commitment to universal, affordable care. But that bipartisan effort became a leading charge in the activists' indictment of Mr. Bennett.

Is there a way to push back against the movement toward partisanship and paralysis -- to carve out some space for those who strive to work across party lines in the national interest? We can think of no more important question, and in the months before the election we intend to provide a forum, on our letters and op-ed pages, to continue the discussion. Is today's polarization part of a normal American cycle or is it a new phenomenon arising from new factors such as cable television, the Internet, geographic self-sorting, campaign finance reform or computer-assisted redistricting? Does it open a space for third parties or other forms of "radical centrism"? If so, would that be good or bad? We'd like to hear your views.

Undoubtedly many Americans are put off by the party puritans; that is one reason they warmed to then-candidate Barack Obama's one-nation message. After his partner in bipartisan crime was defeated, Mr. Wyden told us that he would keep trying. "This is the time when you've got to measure what elected officials are all about," he said. "Don't walk away from the table. Keep looking for ways to bring the country together."

As if to illustrate the point, Mr. Wyden has found a new Republican partner to craft a bipartisan tax reform bill. That's the good news. On the other hand, the partner he found -- conservative New Hampshire Republican Sen. Judd Gregg -- is retiring this year and doesn't have to worry about a primary challenge.